1. Field of the Invention
The performance of a wind machine as related to power output is a function of the efficiency at which the machine extracts energy from the windstream. This phenomenon can be expressed as C.sub.p =P.sub.o /P.sub.w where C.sub.p is the power coefficient or coefficient of performance, P.sub.o is the power output, and P.sub.w is the power in the windstream. Kinetic theory dictates that the maximum value for C.sub.p is 59%. The actual value is in part a function of the airfoil characteristics and configuration. For a given design, C.sub.p is also dependent upon the rotational velocity of the airfoil, measured at its radial extremity or tip, in relation to the free-flow wind velocity. This relationship is commonly referred to as the tip speed/wind speed ratio, or simply the tip speed ratio.
Since the wind velocity at a given site tends to fluctuate, it is apparent that failure of an airfoil to react to such fluctuations would have a substantial impact on the operational efficiency of the machine. This invention relates to a control system for responding to changes in wind velocity so as to maintain a machine's operation at substantially peak performance.
2. Description of the Prior Art
A control system for a vertical axis turbine is taught by F. N. Palma in U.S. Pat. No. 4,168,439. This device is responsive to the prevailing wind speed and is designed to pivot the airfoil blades in order to achieve maximum aerodynamic efficiency. A tachometer is provided to signal the attainment of predetermined speeds for switching from one mode of operation to another. Similarly, in U.S. Pat. No. 4,160,170, Harner et al. shows a system for controlling the blade angle of a wind turbine in response to changes in wind velocity. While the devices of Harner et al. and Palma influence the operational efficiency of their respective wind machines by varying the configuration of the airfoil, they are unable to directly alter tip speed in response to wind speed. Their use is therefore limited to variable pitch turbines.
Moran et al., U.S. Pat. No. 4,095,120, teaches a system adaptable for use with a fixed blade turbine for improving the efficiency of a wind-driven generator. This system includes a generator speed sensor which cooperates with appropriate circuitry for incrementally controlling the field current. In U.S. Pat. No. 3,974,395, Bright also shows a field control system for a wind-driven electrical generator. Bright employs a tachometer coupled to the impeller shaft for generating an output signal representative of wind velocity. This signal is applied to a field control circuit, thereby adjusting the field current of the generator. A limitation characteristic of the systems of Moran et al. and Bright is that the tachometer signal in each case is not necessarily indicative of the actual wind velocity, insofar as the rotational speed of the impeller or shaft is influenced by the load on the generator. This discrepancy results in an inaccurate application of field current for attaining the optimum tip speed ratio.